Re-add markdownlint installation to github actions

Author: IanLee1521

.github/workflows/main.yml

@@ -32,6 +32,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install .[dev]
+          npm install -g markdownlint-cli@0.33.0
 
       - name: Run tests
         run: |